All the data needed by the FUSE clients is contained in the
F_dat structure. This structure contains a
union FD of frequently used type and a buffer
F_databuf to send data to libfuse. The union
contains the following elements:

FD_stat

A struct
statvfs(3) filled in by the
FUSE client statfs for the FUSE VFS statfs code.

FD_attr

Used by the getattr and setattr calls.

FD_io

Contains all fields commonly used by FUSE client callbacks
to provide information to FUSE vnops. It is used by access, readdir,
release, releasedir, read, write, mkdir, and setattr.

Setattr uses a struct fb_io and a struct stat. Settattr uses
FD_stat and encapsulates a struct fb_io in
F_databuf with
fbtod.